Remove references to the current task from task() in stack.yaml
Remove references to the current task from task() in stack.yaml

When using the task() expression function within a mistral task the name
is only needed when referncing a different task. These were found with
the mistral-lint PyPI package.

Change-Id: I8b67323f250fcfcf37a15dcc29024e5ce1506bb4

openstack/tripleo-common,openstack/tripleo-common